The present invention relates generally to electrophoresis apparatus, and more specifically a method and apparatus for the measurement of characteristics such as electrophoretic mobility of particles suspended in a low-permittivity non-aqueous liquid medium. The invention will be discussed in relation to its relevance to electrostatographic printing or copying process parameters but is not so restricted as it also relates to other situations where characteristics such as mobility of particles under the influence of an electric field are of interest.
It is well known that the electrical properties of liquid toners used in electostatography significantly influence the quality of a printed image; the most important electrical characteristics of conventional liquid toners are known to be conductivity, electrophoretic mobility and charge to mass ratio. Although the physical background of these characteristics is understood, accurate and reliable methods for measuring these properties are limited.
The term zeta potential is understood in colloidal chemistry as an indicator of the electrophoretic mobility of particulate matter colloidally suspended or dispersed in a fluid medium. The electrophoretic mobility, in turn, is an indication of the velocity of the particles through the solution due to the effect of an applied electric field.
The stability of a particle suspended in a bulk medium is related to the zeta potential of the particle. Stable particles remain dispersed whereas unstable particles tend to agglomerate and eventually precipitate out of the solution. The higher the zeta potential, the more stable the system is since highly charged particles repel one another and remain dispersed.
In the field of liquid electrostatographic printing and copying, development of a latent image takes place at high speeds, which requires, in one form, that a large amount of uniformly characteristic liquid developer marking particles be supplied to the latent image surface as uniformly as possible to produce a high quality image without any variations in the development thereof.
Therefore, the measurement of the mobility of electrostatic particles dispersed in a dielectric medium under the influence of externally applied electric fields is useful for allowing the assessment of liquid toner sensitivity and at the same time providing a means for analysis of the behaviour of liquid toners as well as their individual constituents.
Various prior art apparatus and means have been utilised to measure the zeta potentials of colloidal particles suspended in a liquid medium. U.S. Pat. No. 3,454,487 to Riddick discloses an electrophoresis apparatus, wherein the electrophoretic mobility, i.e. the velocity of the particles per unit field strength, is measured in an electrophoretic cell which consists of sample receiving chambers connected by a liquid-flow communication passageway.
This measurement is performed utilising an ocular micrometer or distance scale which is inserted into the eyepiece of a microscope. The particles are timed as they cross a fixed distance in the observation chamber of the electrophoretic cell under a D.C. electric field of known strength.
In U.S. Pat. No. 3,764,512 to Greenwood et al. discloses an apparatus utilising a coherent light beam from a laser which is caused to intermittently scan a path located on the stationary layer of an electrophoresis chamber by means of a mirror galvanometer at a rate equal to the migration rate of the particles in the chamber. The operator views the migrating particles in the chamber through a microscope and simultaneously adjusts the scanning rate of the mirror galvanometer by adjusting a potentiometer in the galvanometer control circuit until the scanning laser beam appears to visually track the migrating particles as viewed through the microscope. Via appropriate scaling circuitry interacting with the galvanometer drive circuit and the circuit supplying the voltage drop across the chamber, a value for electrophoretic mobility or zeta potential may automatically be displayed through suitable means, such as an electronically operated digital readout.
In U.S. Pat. No. 3,793,180 to Flower et al. discloses a system capable of measuring zeta potential, particle size distribution, total charge density and other distribution functions of aqueous suspensions. A laser beam is provided in the instrument and is focused on the particles in the sample solution which are contained in an electrophoretic cell. A reticle or grating is positioned such that the reflection of the laser beam from the particles passes through the reticle to a photo-tube placed on the other side of the reticle. Then, as the particles move through the solution the photo-tube is intermittently illuminated through the reticle and, as a result, generates a train of electric pulses whose frequency is directly proportional to the velocity of the particles. Since the frequency of the electrical signal is a measurement of the particle velocity, it is also a measure of zeta potential.
In U.S. Pat. No. 3,909,380 to Day, a television camera or other equivalent image sensor, such as a photosensitive array utilising a suitable lens system, observes the fluid within an electrophoresis cell which is illuminated by the cold light of a fiberoptic source in order to prevent the production of convection currents in the medium. The suspended particles are magnified by a microscope and the image is projected onto the monitor screen. A reference pattern is superimposed onto the monitor and the sweep speed of the reference pattern is then manually adjusted to match the speed of any single particle or group of particles on the monitor screen. The sweep speed of the reference pattern may then be converted to a zeta potential signal which must be corrected for the temperature of the sample.
In U.S. Pat. No. 4,046,667 to Goetz describes an electrophoresis chamber, a circuit for impressing a voltage across the chamber, a light beam to illuminate a portion of the chamber, and a microscope including an objective lens system and an eyepiece for viewing illuminated particles migrating relative to a suspending medium within the chamber under the influence of the applied voltage. Within the microscope, between the objective lens and the eyepiece, is a movable optical prism driven by a galvanometer, the drive circuit of which includes an adjustable potentiometer for controlling the rate and direction of movement of the optical prism.
A circuit connected to the galvanometer drive circuit and the circuit applying the voltage potential across the chamber are adapted to develop a signal proportional to the electrophoretic mobility or zeta potential of the migrating particles when the rate of movement of the optical prism is adjusted such that it cancels the transfer velocity of the migrating particles. The particles then appear stationary when observed through the eyepiece of the microscope.
The hereto disclosed prior art apparatus and means can be characterised as utilising direct methods in which to determine electrophoretic mobility, that is, electrophoretic mobility being a linear function of particle velocity, is therefore derived from direct measurement of actual said velocity; this requiring the liquid toner sample to be transparent or semi-transparent to the light beam of the sensing device. In contrast, there also exist techniques whereby indirect methods can be used in determining electrophoretic mobility. In these indirect methods, secondary effects caused by particle motion within the dispersion are used, processed and converted into mobility measurements, as taught by the following prior art examples.
In U.S. Pat. No. 4,679,439 to Culkin, there is described a method and apparatus for measuring the unsteady sedimentation potential of particles in a suspension comprising the insertion of a portion of the suspension of particles in a cell, the cell having a first and a second electrode. A speaker motor means is used to vibrate the cell and to accelerate the particles in suspension and allowing the measuring of the unsteady sedimentation potential of the particles across the first and second electrodes, the cell being vibrated at a frequency in the range between 0.0001 to 50 kHz.
In U.S. Pat No. 4,928,065 to Lane et al. describes a method and apparatus for classifying non-aqueous liquid suspensions of charged particles employing a large time-varying electric field applied to a suspension situated between capacitive test electrodes. A current waveform is produced that characterises the suspension with respect to critical properties including concentration, mobility, and plating tendencies of the charged particles suspended therein.
In U.S. Pat No. 5,848,322 to Chen et al. discloses an apparatus for determining charge density and mobility in a liquid solution having electrically charged particles therein. The apparatus includes an electrode and a dielectric member having a first surface situated opposite the electrode for providing a volume therebetween in which a sample of the liquid may be placed. A fixed bias voltage applied to the electrode to produce an electrical current flow through the liquid solution and the dielectric member. A device coupled to the dielectric member, measures the electrical current as a function of time to provide a measure of voltage decay across the liquid solution. The voltage decay corresponds to the charge density of the liquid solution.
In U.S. Pat No. 4,497,208 to Oja et al. discloses a method and apparatus for measuring the electro-kinetic properties of a liquid dispersion in which electrodes are placed in the liquid and an apparatus is provided for applying an alternating electrical potential to these electrodes. The electric field from this alternating potential acts upon the charged elements in the liquid, resulting in the generation of sound at the frequency of the applied electrical potential. By placing a conventional acoustic transducer in a spaced relationship to the electrodes, the acoustic signal is detected and measured. The amplitude of the acoustical signal will be a function of the electro-kinetic properties of the particles in the liquid. In its preferred form, the electrodes that are placed in the liquid have a spacial separation of one-half wave length, or odd integer multiples of half wave length of the sound which they generate. Coupled to the receiving transducer is a receiver that will amplify the signal to convenient levels.
In U.S. Pat No. 5,059,909 to O""Brien also discloses a measurement device whereby the interaction of sound waves and electric fields in the fluid medium over a range of frequencies are used to obtain the particle size and zeta potential.
The presently preferred indirect electrophoretic mobility measurement is the so called electro-kinetic sonic amplitude (ESA) technique. This method however, uses very high frequency AC signals and relies to a great extent on a number of theoretical principles in order to convert the measured ESA signal into electrophoretic mobility values. It is difficult to directly apply the results of these high frequency measurements to the actual DC electrophoresis of liquid toner marking particles, especially in relation to image development in a typical electrostatographic process.
It can be realised from all the above prior art discussion that the requirements for the measurement of the mobility of marking particles in a liquid of high viscosity, in the order of up to 10,000 mPaxc2x7s, and of high solids content, of up to 60% by weight, for electrostatographic methods can not be readily and accurately determined by any of the hitherto described art. The above described apparatus and methods, referred to herein as direct measurement techniques, have been primarily designed for liquid mediums which are to a degree transparent or semi-transparent such that an observer or means for detecting the movement of individual particles is possible, or utilise techniques and derive results which cannot be readily applied to electrostatographic development processes being currently disclosed. Also, these prior art methods are normally associated with liquids which can be defined in terms of conventional electrostatography as low viscosity liquids. The prior art does not fulfil the requirement of duplicating real electrographic process parameters such as for example, utilising actual gap dimensions and appropriate electric field strengths.
Further, it is well known that marking particle dispersions of xe2x80x9cworking strengthxe2x80x9d dilution, that is, particle dispersions with approximately 0.5 to 2% by weight solids content, require in most cases further dilution of as much as 100:1 to allow measurements to be undertaken in most prior art apparatus of the type described.
There is a need therefore, to be able to characterise liquid toners which have a range of viscosity, and a range of concentration without the need for dilution, and in which the replication of real conditions, such as actual gap dimensions, associated with printing or copying system are realised.
It is an object of the instant invention to provide an apparatus and method whereby synchronous optical and electrical measurements, which can provide complimentary information about a particles mobility and charge as well as other specific characterisation of working strength marking particle dispersions, can be undertaken, the dispersions of marking particles consisting of liquids having a viscosity, in the order of up to 10,000 mPaxc2x7s, and a solids content of the marking particles, of up to 60% by weight therein. Such a marking particle dispersions having a total viscosity in the order of up to 500,000 mPaxc2x7s.
The present invention describes an apparatus and method that allows synchronous electrical and optical measurements of the mobility of concentrated particles suspended in a liquid medium of high viscosity.
The present invention discloses novel apparatus and method for characterisation of pertinent properties of low permittivity non-aqueous liquid dispersions of charged particles, including the electrophoretic mobility, toner compaction and other pertinent electrical parameters deemed of importance to liquid toners in the art of electrostatography and which significantly influence the quality of a printed image.
In one form therefore the invention is said to reside in a toner characterisation cell adapted to determine characteristics of insoluble particles in a liquid medium, the cell including a first electrode and a second electrode spaced apart by a selected gap, means to provide an electric field between the first electrode and the second electrode, means to measure a displacement current between the first electrode and the second electrode, first and second optical density measuring devices adapted to measure the change in optical density in the cell adjacent each electrode, and means to determine the characteristics from the displacement current and the change in optical density.
Preferably the first electrode and the second electrode are substantially transparent or translucent. To allow for transparency or translucency while still acting as an electrode the electrodes may be coated with a transparent conductive material. For instance the transparent conductive material may be conductive indium tin oxide. The indium tin oxide may be coated on a front surface of the electrodes.
The electrodes may be substantially planar and may be formed from glass plates.
Where the cell is used for the characterisation of toners and the like in which the gap is substantial there may be used a wall arrangement to contain the toner between the electrodes. The gap between the electrodes may be in the range of 1 xcexcm to 10 mm.
There may be used micrometer means to adjust and measure the gap between the first electrode and the second electrode.
The optical density measuring devices may each comprise an infra-red emitting diode and phototransistor pair.
The composition which comprises the insoluble particle in a liquid medium characterised according to this invention may be a liquid toner for electrostatography or an ink or other particle suspension. Such a liquid toner or ink may be of low or high viscosity.
In particular the invention is useful for the characterisation of insoluble particle suspensions in a liquid medium where there is to be an electric field applied, particularly a high voltage electric field.
In an alternative form the invention may be said to reside in a cell including a base, a tool post mounted to the base, a longitudinal track on the tool post, a traveller on the track, a first electrode assembly mounted to the base, a second electrode assembly mounted to the traveller, displacement adjustment means between the post and the traveller whereby the distance between the first electrode assembly and the second electrode assembly may be adjusted, a first optical density measuring device associated with the first electrode assembly, a second optical density measuring device associated with the second electrode, means to provide an electric field between the first electrode and the second electrode, means to determine a displacement current between the first electrode and the second electrode, and means to calculate electrophoretic characteristics of a suspension of particles in a highly viscous dispersion placed between the first and second electrodes from the displacement current and the change in optical density at the first and/or the second electrodes determined with the optical density measuring devices.
In an alternative form the invention may be said to reside in a method of determining the characteristics of particles in a dispersion, a method including the steps of causing particles to move in response to an applied electric field between a first electrode and a second electrode spaced apart by a selected distance, determining a displacement current caused by the particles so moved, determining by optical means particle compaction or diffusion due to the particle movement adjacent the first and/or second electrodes and determining the characteristics from the displacement current and the change in particle compaction or diffusion.
In an alternative form the invention may be said to reside in a method of determining the mobility of particles in a liquid dispersion, the method including the steps of placing a selected amount of the liquid with the particles suspended therein on to a first planar transparent electrode, moving a second planar transparent electrode adjacent the first electrode such that they are spaced apart by a selected gap, applying a selected voltage to one of the electrodes to cause particles to move electrophoretically between the first electrode and the second electrode, measuring a displacement current caused by the particles so moved, determining by optical means particle compaction or diffusion due to the particle movement adjacent the first and/or second electrode and calculating the particle mobility from the displacement current and the observed changes in optical density.
The data collected from both the optical means such as phototransistor detectors and the displacement current measuring means such as power supply and current sensing means, may be fed to a computer allowing mathematical manipulation of the data and therefore permitting characterisation of the liquid toner dispersion including determination of particle mobility and the compaction ability of the marking particles within said dispersion.
The device and method of the present invention may be used in a laboratory for the characterisation of insoluble particle suspensions in a liquid media in either development or manufacturing settings. Alternatively the device and method of the present invention may be used for process control, built into for instance a electrostatographic printing device, to monitor the use of toners or inks, for instance, during recycling of toners, and as such could incorporate a particle suspension flow through means.
In conventional liquid electrostatographic printing and copying, the characterisation of dispersed electrostatic particles by means of electrophoretic analysis plays an important role in predicting the quality of liquid developers as well as allowing the replication of real development condition in such a system.
Electrokinetic potential is understood in colloidal chemistry as an indication of the electrophoretic mobility of particulate matter suspended or dispersed in a fluid suspending medium, and as an indication of the velocity or migration of a colloid particle through a fluid suspending medium under the effect of an applied electric field. It is understood that the suspended particles are considered electrically neutral, in the sense that they appear to have little if any activity with regard to an electric charge measuring means, but have an electrically charged double layer of electrons at the surface of the particles. That is, with suspended particulate material in a fluid medium, each particle is surrounded by a double layer of electrical or electrostatic charges, the inner layer of which may be considered immediately adjacent or fixed to the particle, while the outer layer of charges may relate more to constituents or characteristics of the suspending medium and to posses a charge opposite to that of the inner layer.
The net electric charge surrounding the particle, therefore, may be substantially zero with a resulting electric neutrality of the particle, yet as is now understood, such double layer of electric charges or xe2x80x9cdiffuse double layerxe2x80x9d produces a colloidal and or interfacial effect on the mobility or occluding tendency of the particles and or the electrokinetic potential or electrophoretic mobility thereof in the suspending medium. The electrokinetic potential or zeta potential may be conveniently considered as the electrical potential drop across the diffuse double layer of electric charges at the interface between the surface of the particle and the bulk of the suspending medium.
The measurement of the zeta potential then gives an indication and useful information with regard to the electrophoretic or electrokinetic movement or migration or mobility of colloidal or other larger dispersed particles through a suspending medium.
In general, a liquid toner for developing electrostatic images is prepared by dispersing an inorganic or organic colorant such as iron oxide, carbon black, nigrosine, phthalocyanine blue, benzidine yellow, quinacridone pink and the like into a non-conductive dielectric liquid vehicle which may contain dissolved or dispersed therein synthetic or naturally occurring polymers such as acrylics and their copolymers, alkyds, rosins, rosin esters, epoxies, polyvinyl acetate, styrene-butadiene, cyclised rubber, ethylene vinyl acetate copolymers, polyethylene, etc. Additionally, to impart or enhance an electrostatic charge on such dispersed particles, additives known as charge directors or charge control agents may be included. Such materials can be metallic soaps, fatty acids or lecithin.
It can be appreciated from the foregoing therefore, that the immediate response of such a liquid toner to an externally applied electric field within an electrophoretic cell arrangement, is that although all charged species in the liquid toner dispersion will react, it is the highly mobile species that are rapidly swept to an oppositely charged electrode. As the spatial displacement of any charge constitutes an electric current, a significant initial current is therefore measured, without any significant toner marking particle deposition or movement. The magnitude of this current is dependent upon the motion of these highly mobile species, since they are the major charge carriers in the liquid toner dispersion. Once these charge carriers are swept from the liquid, the less mobile toner marking particles become the major contributors of charge transfer, resulting in a drop of the measured current but an increase in the deposition of marking particles on the oppositely charged electrode.
It is to be understood that current transient readings enable the measurement of the total displacement current created by all individual charge carriers. Due to the fact that there always exists a velocity distribution between the charged species experiencing electrophoretic migration, it often becomes difficult to distinguish the contribution of toner marking particles from other charge carriers.
It can be readily acknowledged therefore, the requirement to simultaneously capture both optical and electrical displacements which provide complimentary information about a particles mobility and charge leading to the specific characterisation of a marking particle dispersions as it relates to electrostatic latent image development.
In the case where the liquid toner sample is not of a transparent nature or of a useable viscosity, the prior art methods require the toner sample to be transparent to light and therefore would need to be diluted to carry out any particle mobility measurement. The act of dilution leads to mechanical and electrical sample changes which in turn affect the behaviour of the sample under an electric field and therefore giving an erroneous result which would not be indicative of the liquid toners performance in an electrographic printing or copying system.
Further, some examples of liquid toners can be characterised by particle mobility, of about 10xe2x88x9211 to 10xe2x88x9212 m2/V/s, that is, substantially lower than that of conventional liquid toners and approaching the detectable limits of most mobility measuring methods. Also, the printing or copying process electric fields associated with these new types of liquid toners are normally greater than 106 V/m; high fields are required to compensate for the low electrophoretic mobility of the particles within these liquid toners. The current invention provides a means whereby undiluted liquid toners of the type described can be easily characterised.
For an understanding of the features of the present invention, reference is made to the drawings, wherein like reference numerals have been used throughout to designate identical elements. It will become apparent from the following discussion that the apparatus of the present invention may be modified to suit a wide variety of embodiments; thus although the present invention will be described in connection with a preferred environment thereof, it will be understood that the description of the invention is not intended to limit the invention to this preferred environment and or embodiment. Indeed the description is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims of the present invention.